(i) Field of the Invention
This invention is concerned with processes for dealkalization or acidification of aqueous salt solutions, as well as for the splitting of the salt of such solutions employing a water splitting system.
More especially the invention relates to the dealkalization and/or acidification of alkaline or non-alkaline aqueous solutions of various compositions; and to the dealkalization and/or splitting of the salt of alkaline or non-alkaline aqueous solutions of various compositions.
(ii) Description of Prior Art
The simplest case involves the partial dealkalization of an alkaline but otherwise pure solution of water to a minimum conductivity dictated by the efficient operation of a 2-compartment or 3-compartment water splitter. Assuming that sodium hydroxide is the base to be removed the minimum operational conductivity is about 20 mS/cm and the corresponding concentration of sodium hydroxide is about 0.1M. For a better efficiency in terms of power consumption a conductivity of 30 mS/cm corresponding to a concentration of NaOH of 0.15M is recommended. The cation of the base could be the type that does not hydrolyze (e.g. Na+) or the type that does (e.g. NH.sub.4 +).
In the case of a 3-compartment water splitter the co-products are sodium hydroxide (base compartment) and water (acid compartment). In order to maintain conductivity in the acid compartment an electrolyte (acid, base or salt) should be added of a concentration sufficient to maintain a minimum conductivity of 20 mS/cm. This solution can be continuously recirculated through the acid loop.
A second case in a first embodiment involves the partial or complete dealkalization and/or acidification of an alkaline or non-alkaline solution of a salt of the type whose cation and anion do not hydrolyze in water (e.g. NaCl) using a 2-compartment water splitter. In this case the removal of alkali cations can continue beyond the complete dealkalization point thus acidifying the aqueous salt solution by replacing the displaced alkali cations with hydrogen ions; this process is covered in U.S. Pat. No. 4,391,680 by Mani and Chlanda. The acidification process in this case can continue until completion since the conductive salt is being replaced by an acid which is even more conductive than the salt itself.
A second embodiment of the second case involves the partial or complete dealkalization and/or splitting of the salt of an alkaline or non-alkaline solution of a salt of the type whose cation and anion do not hydrolyze in water (e.g. NaCl)using a 3-compartment water splitter. In this case since the hydroxyl anions are more mobile and in addition are better bases than any other ions they would be expected to preferentially migrate to the acid compartment over the competing anions of the salt. Depending on the relative migration rates of hydroxyl and the anion of the salt, the salt may have to be partially depleted before being completely dealkalized. As in case 1 an electrolyte may have to be added to the acid compartment in order to maintain conductivity in the system. Electrohydrolysis can continue beyond the complete dealkalization point thus splitting the salt into its corresponding acid (e.g. HCl) and base (e.g. NaOH). This process which has been covered in U.S. Pat. No. 2,829,095 to Oda et al, can efficiently continue until the conductivity of the salt compartment reaches about 20 ms/cm.
A third case in a first embodiment involves the partial or complete dealkalization and/or acidification of an alkaline or non-alkaline solution of a salt of the type whose cation does not hydrolyze but whose anion does to produce hydroxyl ions, (e.g. NaHO.sub.2)using a 2-compartment water splitter. In this case the alkali cation removal process can continue beyond the complete dealkalization point thus modifying the aqueous salt solution by replacing the displaced alkali cations with hydrogen cations. In this case, however, the extent of alkali cation removal is limited by the degree of ionization of the acid produced, which is usually very weak. Since, during the alkali cation removal process the original salt is being replaced by a very weak acid, this process can only efficiently continue as long as the decreasing conductivity of the depleted salt solution remains above 20 mS/cm.
A second embodiment of the third case involves the partial or complete dealkalization and/or splitting of the salt of an alkaline or non-alkaline solution of a salt of the type whose cation does not hydrolyze but whose anion does to produce hydroxyl ions, (e.g. NaHO.sub.2) using a 3-compartment water splitter. In this case the alkali cations (e.g. Na.sup.+) migrate to the base compartment in which alkali metal and/or non-metal hydroxide (e.g. NaOH) is formed whereas the hydroxyl anions as well as the anions of the salt (e.g. HO.sub.2 -) migrate to the acid compartment in which water and the acid of the anion of the salt (e.g. H.sub.2 O.sub.2) form. Since, however, both the hydroxyl anions and the anions of the salt migrate simultaneously to the acid compartment it is not possible to remove all caustic from the salt compartment without deleting the salt itself. Another problem is that none of the compounds forming in the acid compartment (e.g. H.sub.2 O and H.sub.2 O.sub.2) are conductive. The solution to both of these problems is the addition of alkali metal and/or non-metal hydroxide (e.g. NaOH) electrolyte into the acid compartment. In this way the formation of the acid of the anion of the salt (e.g. H.sub.2 O.sub.2) is avoided and instead the salt of the anion is formed (e.g. NaHO.sub.2). Thus, the desired ratio of alkali metal and/or non-metal hydroxide to salt of the migrating anion can be achieved in the acid loop by adjusting the original concentration of alkali metal and/or non-metal hydroxide in this loop and/or the extent of electrohydrolysis.
A fourth case involves the partial or complete dealkalization and/or acidification and/or splitting of the salt of an alkaline or non-alkaline solution of a salt of the type whose anion does not hydrolyze but whose cation does to produce hydrogen ions (e.g. NH.sub.4 Cl) In the presence of a hydroxide of an alkali metal and/or non-metal (e.g. NaOH) the hydrolysis equilibrium of the cation shifts completely to the right, e.g. EQU NH.sub.4 Cl+NaOH.revreaction.NH.sub.3 +NaCl+H.sub.2 O
thus forming a solution of alkali metal chloride; its dealkalization and/or acidification and/or splitting of the salt is, therefore expected to be as in case 2. Since the anion is of the type that does not hydrolyze the acidification is expected to proceed as in case 2. The process can continue until the conductivity in the salt compartment is about 20 mS/cm.
A fifth case involves the partial or complete dealkalization and/or acidification and/or splitting of the salt of an alkaline or non-alkaline solution of a salt of the type whose cation as well as anion hydrolyze to produce hydroxyl and hydrogen ions respectively, (e.g. NH.sub.4 HO.sub.2) The initial pH of the solution of such a salt will depend on the relative hydrolysis constants of the cation and anion. In an alkaline solution of a salt of this type the hydrolysis of the anion is suppressed and the hydrolysis of the cation is carried to completion. This suggests that the dealkalization of such solutions will proceed as in case 2. As the dealkalization proceeds, however, the hydrolysis of the cation will increasingly be suppressed and the hydrolysis of the anion would be carried closer to completion; therefore, the latter part of the dealkalization process and the splitting of such salts is likely to proceed as in case 3. In an acidic solution of the salt in question the hydrolysis of the cation would be completely suppressed and the hydrolysis of the anion would be carried to completion; therefore the acidification of such solutions will proceed as in case 3.
The sixth case involves alkaline or nonalkaline solutions of the types discussed in cases 1 to 5 above, but also containing water soluble but non-conductive compounds (e.g. ethanol) and/or insoluble but suspended compounds (e.g. Mg(OH).sub.2 colloidal particles).
In all of the cases described above employing a 3-compartment water splitter the co-products are the hydroxide of the alkali metal and/or non-metal cation that was removed from the original solution and the acid of the anion of the salt.
A problem exists in processes for the complete dealkalization of an alkaline monosodium peroxide (NaHO.sub.2) solution. Solutions such as these are produced by the reduction of oxygen in electrolytic cells employing sodium hydroxide as the electrolyte in the anode compartment, (e.g. the Dow on-site peroxide generator, U.S. Pat. Nos. 4,224,129 and 4,317,704). Since, however, completely dealkalized solutions of monosodium peroxide are needed for the efficient bleaching of mechanical pulps the need exists for the dealkalization of these solutions. Alternate approaches, such as acidification of the solution from an external source, consume the acid added, waste caustic soda and furthermore change the nature of the solution, since a new salt is formed as a result of the neutralization reaction.
Approaches other than the addition of acid from an external source, for the dealkalization and/or acidification of aqueous solutions, include: electrolytic systems (e.g. U.S. Pat. No. 4,671,863 by Tejeda), ion-exchange systems employing strong-acid cation resins, weak-acid cation resins, and anion resins (e.g. McGarvey, F. X., Power, 128(8), 59-60, 1984), systems employing magnetic ion-exchange resins in fluidized bed adsorbers (e.g. Bolto et al, J. Chem. Technol. Biotechnol., 29(6), 325-31, 1979) and electrodialytic systems (e.g. U.S. Pat. No. 3,893,901 by Tejeda). The ion-exchange systems referred to above are primarily intended for the removal of trace amounts of alkali from water for purification purposes; the scale-up of such systems, for the removal of large quantities of alkali from industrial streams, would be uneconomical because of the high cost associated with the regeneration of ion-exchange columns. In addition, in such systems, alkali is obtained in the form of a salt and not caustic. Electrodialytic systems, on the other hand, recover the alkali in the form of caustic, the energy costs associated with dealkalization, however, are significantly higher than those associated with water splitting techniques.
Membrane systems involving stacked pairs of membranes have been recommended for various applications. These include desalination (U.S. Pat. No. 3,654,125 to Leitz), springing of sulfur dioxide from aqueous sulphite and bisulfite solutions (U.S. Pat. No. 4,082,835 to Chlanda et al), the removal of alkali metal cations from aqueous alkali metal chloride solutions so as to produce an acidified salt solution and sodium hydroxide (U.S. Pat. No. 4,391,680 to Mani and Chlanda) and the recovery of valuable metal or ammonium values from materials comprising a salt of a first acid while avoiding the formation of gas bubbles (U.S. Pat. No. 4,592,817 to Chlanda and Mani). In none of the aforementioned systems, however, suggestion is made for their application to the partial or complete dealkalization of alkaline alkali metal and/or non-metal salt solutions. Furthermore, for only one type of salt (case 2) reference is made to the acidification of solutions of salts (U.S. Pat. No. 4,592,817 to Chlanda and Mani).
Membrane systems involving water splitters in the three-compartment configuration have been recommended for various applications. These include the recovery of fluorine values from fluorosilic acid aqueous streams by electrodialytic water splitting of fluoride salt to hydrofluoric acid and hydroxide base (U.S. Pat. No. 3,787,304 to Chlanda et al), the recovery of TiO.sub.2 from ilmenite-type ores by digestion with hydrofluoric acid, in which hydrofluoric acid and ammonium hydroxide are recovered by an electrodialytic water-splitting process from by-product aqueous ammonium fluoride (U.S. Pat. No. 4,107,264 by Nagasubramanian and Liu), the conversion of alkali metal sulfate values, such as sodium or potassium values in spent rayon spin bath liquors, into alkali metal hydroxide and alkali metal sulfate/sulfuric acid (U.S Pat. No. 4,504,373 by Mani and Chlanda) and the recovery of metal or ammonium values from materials comprising a salt of a first acid while avoiding formation of gas bubbles in the electrohydrolysis cells. In none of the aforementioned systems, however, suggestion is made for their application to the partial or complete dealkalization of alkaline alkali metal and/or non-metal salt solutions; furthermore, no attempt is made to cover the splitting of the dealkalized or neutral salts of various types as described above.